The source revealed the bomber, 22, was among a ‘larger pool’ of former ‘subjects of interest’ on MI5’s radar and the risk he posed was subject to review.

Deciding whether to monitor a person around the clock relies on ‘difficult professional judgments based on partial information’, the source stressed.

Officials said 18 plots had been thwarted since 2013. Five had been disrupted since the Westminster attack on March 22, compared with 13 in the three years before.

As the security services faced questions about how the Manchester bomber was able to slip through the net, a Whitehall source pointed to the ‘unprecedented scale of threat’.

Britain’s security services are considered among the world’s best but are struggling to monitor the flood of potential terrorists returning from Syria and Iraq.

Of the 304 arrests, 108 resulted in a charge, including 91 suspects charged with terror-related offences.

One hundred people were released without charge, 88 were bailed pending further investigation and eight faced ‘alternative action’.

The tally for 2016/17 includes 12 arrests made as part of the investigation into the Westminster attack in March, which saw terrorist Khalid Masood mow down people on Westminster Bridge before stabbing police officer Keith Palmer to death.

All those arrested were released without charge and told they would face no further action over the attack which saw six people killed, including attacker Masood.

Since the Westminster atrocity there have been two more deadly attacks, in Manchester and London Bridge, while counter-terror agencies have foiled five alleged plots.

The scale of the threat facing the country has been laid bare with figures showing police and MI5 are involved in 500 investigations involving 3,000 individuals at any one time.

There are also 20,000 former ‘subjects of interest’ whose risk must be kept under review.